Prime Minister Menachem Begin has sent President Reagan a strongly worded and detailed condemnation of a Saudi Arabian peace plan for the Middle East and warned that U.S. expressions of interest in the plan could impede the Camp David process.

Begin briefed the Israeli Cabinet today on the letter, in which the prime minister complained that the United States is endangering the prospects of Middle East peace by encouraging a Saudi role.

The letter, reportedly written Friday and sent to the White House this weekend, represents an intensification by Israel of pressure on the Reagan administration to distance itself from the eight-point peace plan proposed by Saudi Crown Prince Fahd in August. A separate protest was made to Secretary of State Alexander M. Haig Jr. on Friday by Ephraim Evron, Israel's ambassador to the United States.

The plan has drawn support in Europe, and Reagan and Haig spoke warmly of some of its provisions last Thursday but said there were problems with other aspects of it.

Begin, interviewed from here for the ABC program "Issues and Answers," said today that the Saudi plan is an example of "how to liquidate Israel in stages." He said Israel felt threatened by what seems to be increasing support in the United States and Europe for elements of the plan.

He is expected to turn again to the subject Monday in what is being billed as a major political speech to the opening of the winter session of the Israeli parliament.

Meanwhile, in what some observers saw as a related move, Israeli Defense Minister Ariel Sharon reportedly has informed the Reagan administration that he is postponing a visit to Washington scheduled for later this month in which he was to have discussed strategic cooperation between the United States and Israel. The reason reportedly given was Sharon's heavy schedule.

The contents of Begin's letter to Reagan were not disclosed, but a Begin aide said it "deals in detail with the prime minister's point of view on the entire Saudi Arabia subject. He tells the president what he thinks about the Saudis, about the so-called peace plan and the United States' reaction to it."

Begin is understood to have told Reagan that the Saudi proposals -- which call for Israeli withdrawal from all occupied territory, the establishment of an independent Palestinian state in the West Bank and Gaza Strip and an affirmation of the right of "the states in the region" to live in peace -- is nothing more than a compendium of extremist Arab resolutions aimed at the destruction of Israel.

Similar expressions of concern were made by Evron in Washington Friday. Evron's complaint apparently was prompted by State Department insistence that the Saudi plan implicitly recognizes Israel's right to exist.

Israeli government sources said that the Begin letter emphasizes that the Saudi plan contains no mention of the need to conduct negotiations with Israel -- nor even mentions Israel by name -- precisely because to do so would imply recognition.

Moreover, Begin is understood to have written that there is no mention in the Saudi plan of U.N. resolutions 242 and 338, which call for negotiations with Israel and, therefore, imply recognition of the Jewish state. Both resolutions also call for Israeli withdrawal from territories occupied in the 1967 war, a demand that Israel has rejected.

In analyzing the Saudi proposals, Israeli officials today pointed to a clause calling for a transitional period in which the West Bank and Gaza Strip would come under U.N. supervision pending establishment of a Palestinian state. This clause, the officials said, is designed to avoid any direct Saudi recognition of Israel. Begin is understood to have made that point to Reagan.

Israeli Foreign Ministry analysts said Saudi Arabia made the peace proposals public last summer, when Saudi officials were acting as intermediaries in negotiations for an Israeli and Palestine Liberation Organization cease-fire in southern Lebanon and when it appeared to so-called "rejectionist" Arab states that the Saudis were getting too close to recognition of Israel. The Saudi peace plan "was intended to show that Saudi Arabia is sticking to its old, rejectionist positions," one analyst said.

Concurrently, the officials said, the proposals were issued when the Reagan administration was talking encouragingly about the Camp David process and were intended to stop that peace initiative. Begin's letter appeared to be part of a coordinated campaign by Israel to mute U.S. expressions of interest in the Saudi plan and direct the focus to the Camp David accords.

This theme was taken up Friday by Foreign Minister Yitzhak Shamir, who said that each of the Saudi proposals represents a "poisoned dagger thrust into the heart of Israel's existence," and by Defense Minister Sharon, who repeated his allegation that the United States is indirectly providing sophisticated weapons to Iraq by arming Saudi Arabia and Jordan. The United States has denied that charge, but defense sources here have responded that proof of arms transfers by Jordan and Saudi Arabia to Iraq could be made public by Israel.

The prime minister's office said today it had no comment on reports of a secret U.S.-Saudi agreement in which the United States would position military equipment in Saudi Arabia and build a sophisticated communications and command network there for use by U.S. troops in the event of a Persian Gulf crisis.

Uri Porat, Begin's press secretary, said, "You can't expect the prime minister to react to something that has not been confirmed." However, a senior adviser to Begin said, "If this story is true, the only word that comes to mind is 'fantastic.' I hope this report is wrong."